1. Field of the Invention
The present invention relates to a robot system including a robot with a transfer device for transferring a workpiece.
2. Description of the Related Art
When transferring a workpiece between pressing machines, a robot for transferring the workpiece is installed between the pressing machines. In this case, when the space between the pressing machines is large, a transfer device is attached to the wrist of the robot and the transfer device and robot arm transfer the workpiece by their operation. An example of the related art of such a transfer device is disclosed in Japanese Utility Model Laid-Open Publication Hei4-42390, xe2x80x9cArticle Transfer Devicexe2x80x9d.
The transfer device of the related art has a slider and holding means for holding a workpiece with absorbing force and moving along the slider. To transfer the workpiece, the robot moves the transfer device to the pressing machine on one side first and the holding means arranged at one end of the slider adsorbs and holds the workpiece of the pressing machine on one side. Next, the robot slides the holding means together with the workpiece to the other end of the slider and moves the transfer device to the pressing machine on the other side. When the workpiece reaches the pressing machine on the other side, the robot releases the workpiece from the holding means. In this way, even when the transfer distance is long, a workpiece can be transferred.
In the aforementioned conventional transfer device, there is a problem that the transfer speed is inevitably slow because the transfer device uses a screw thread to slide the holding means.
An object of the present invention is to provide a robot system having a faster transfer speed than that of a conventional robot system with the holding means which is slid by a screw thread.
According to the present invention, a robot system including a 6-axis robot, includes: a swingable arm for transferring a workpiece between a first position and a second position, a wrist mounted on a distal end of said arm, said wrist being driven to rotate around a predetermined axis, and a transfer device disposed at said wrist, wherein said transfer device has a link mechanism having a first link and a second link and holding means attached to a distal end of said second link, wherein a proximal end of said first link is attached to said wrist so as to rotate around said predetermined axis, wherein a distal end of said first link and a proximal end of said second link are rotatably connected to each other, and wherein, in accordance with a swinging motion of said arm, said first link rotates in a first rotational direction with respect to said arm and said second link rotates in a second rotational direction opposite to said first rotational direction with respect to said first link, so as to transfer the workpiece along a straight line linking said first position and said second position.
Preferably, said proximal end of said first link is fixed to said wrist and said first link rotates with respect to said arm by driving said wrist to rotate.
Preferably, the robot system further includes rotation transmitting means for transmitting a rotation of said first link with respect to said arm to said second link, thereby rotating said second link with respect to said first link.
Preferably, assuming a swinging angle of said arm from a central position between said first position and said second position as xcex1, said wrist is controlled to rotate so that a rotational angle of said first link with respect to said arm becomes (90xe2x88x92xcex1)/xcex1 times of said swinging angle xcex1 of said arm, and said rotation transmitting means is set so that a rotational angle of said second link with respect to said first link becomes 180/(90xe2x88x92xcex1) times of said rotational angle of said first link with respect to said arm.
Preferably, said holding means is rotatably disposed at said distal end of said second link and said holding means rotates so that a direction of said workpiece held by said holding means in an absolute coordinate system is always kept constant by setting a rotational angle of said holding means with respect to said second link to become xc2xd times of said rotational angle of said second link with respect to said first link.
Preferably, the robot system further includes link rotation means having a seventh axis for driving said link mechanism, wherein, when transferring said workpiece, said link rotation means drives one of said first link and said second link to rotate and said wrist drives the other of said first link and said second link to rotate.
Preferably, assuming a swinging angle of said arm from a central position between said first position and said second position as xcex8, a rotational angle of said first link with respect to said arm as xcex81, and a rotational angle of said second link with respect to said first link as xcex82, said link rotation means drives either of said first link and said second link to rotate so that xcex82=2(xcex8+xcex81) is held.
Preferably, said holding means is rotatably disposed at said distal end of said second link and said holding means rotates so that a direction of said workpiece held by said holding means in an absolute coordinate system is always kept constant by setting a rotational angle of said holding means with respect to said second link to become xc2xd times of said rotational angle of said second link with respect to said first link.
Preferably, when said wrist is moved along said straight line at a constant speed to transfer said workpiece, rotational speeds of said first and second links of said link mechanism can be controlled independently of a rotational speed of said wrist.
Preferably, the robot system further includes a base fixed to said wrist and link rotation means having a seventh axis for driving said link mechanism, wherein said proximal end of said first link is rotatably disposed at said base, and wherein, when transferring said workpiece, said link rotation means drives said first link to rotate with respect to said base and a rotation of said first link with respect to said base is transmitted to said second link, thereby rotating said second link.
Preferably, said wrist controls a rotation of said base so that a direction of said base in an absolute coordinate system is always kept constant during a transfer of said workpiece.
Preferably, when said wrist is moved along said straight line at a constant speed to transfer said workpiece, rotational speeds of said first and second links of said link mechanism can be controlled independently of a rotational speed of said wrist.
Preferably, when transferring said workpiece, said wrist is moved at a constant speed and a rotational speed of said first link with respect to said arm when said wrist is in a neighborhood of said first position and said second position is lower than the rotational speed when said wrist is in a neighborhood of a central position between said first position and said second position.
Preferably, when transferring said workpiece, a moving speed of said wrist when said wrist is in a neighborhood of said first position and said second position is higher than the moving speed when said wrist is in a neighborhood of a central position between said first position and said second position and thereby a rotational speed of said first link with respect to said arm is kept substantially constant.
Preferably, said link mechanism comprises at least three links including said first link and said second link, said at least three links being connected in series.
According to the present invention having the aforementioned constitution, the link mechanism is installed at the distal end of the arm and a workpiece can be transferred by the swinging operation of the arm and the operation of the link mechanism, so that, compared with the conventional robot system with the holding means slid by a screw thread, the transfer speed can be increased drastically. Further, the link mechanism can be lightened easily and the load on the arm can be reduced, thus the dynamic characteristic of the robot system can be improved.
Further, if the link mechanism is structured so as to be driven by the wrist, an additional drive motor is not necessary for driving the link mechanism, and there is no need to control an additional drive motor separately from the 6-axis robot motor, and the control of the robot system can be simplified.
Further, if the link mechanism is structured so as to be driven by the link rotation means having the seventh axis, the rotational speeds of the first and second links of the link mechanism can be controlled individually.
Further, if the direction of a workpiece held by the holding means in the absolute coordinate system is always kept constant during transfer, the workpiece can be transferred between the first position and the second position without being inverted.
Further, when the wrist is in the neighborhood of the first position and second position, the rotational speeds of the first and second links are made lower, thereby the vibration of the first and second links can be reduced and the workpiece stop precision can be improved. In the neighborhood of the central position between the first position and the second position, since the first link and second link lie on top of one another with high rigidity, even if the rotational speeds of the first and second links are high, the link mechanism can sufficiently withstand the reaction force caused by rotation.
Further, since the workpiece is transferred along a straight line, the workpiece can be free from the centrifugal force and the transfer route can be minimized.